This invention relates to a drafting instrument for forming circles.
The prior art provides several different circle drawing devices. U.S. Pat. No. 1,154,673 describes a drafting implement which is comprised of a triangle and protractor combination. The protractor is rotatably positioned within the triangle. A plurality of openings within the protractor enable circles of varying diameters to be drawn by inserting a pen or pencil into a hole and by revolving the protector within the triangle. U.S. Pat. No. 4,353,166 describes a device for drawing circles wherein a disk is rotatably carried on a sheet. The disk and sheet each have a series of holes spaced from a center point of the disc into which the tip of a suitable writing instrument may be placed to rotate the disc or sheet with respect to the other to inscribe a circle on a desired surface. U.S. Pat. No. 3,460,261 describes a drafting device which includes a support member and a rotatable member positioned within the support member. To draw circles, the rotatable member is removed from the support member. A pointed instrument may then be inserted into one of the apertures within the rotatable member to serve as the center of the circle and a pencil point may be inserted in any other selected aperture for drawing a circle by the rotating member. U.S. Pat. No. 3,465,445 describes a drawing and design apparatus for drawing circles and other geometric shapes. However, these devices do not allow intricate size selection, and are not suitable for a precision drafting instrument. Most of the devices require the use of a sharp center point which damages the surface to be marked.